Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. In a color display, each pixel includes light emitters that emit light of different colors, such as red, green, and blue. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode displays rely on passing current through a layer of organic material that glows in response to the current. Displays are typically controlled with either a passive-matrix (PM) control employing electronic circuitry external to the display substrate or an active-matrix (AM) control employing electronic circuitry formed directly on the display substrate and associated with each light-emitting element. Both active- and passive-matrix-controlled OLED and LC displays are available. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066.
The amount of light emitted from an LCD is determined by the brightness of the backlight, the transmissivity of the liquid crystals, and the area of the display through which light is emitted. A larger pixel area will transmit more light than a smaller pixel area. Hence, to achieve a desirably bright LCD, the pixel areas are preferably large. In contrast, the brightness of an OLED display depends on the current density passed through the OLED pixels. At higher current densities, brightness is increased and lifetime is decreased. Thus, a larger light-emitting OLED area will increase the lifetime of an OLED display by reducing the current density or enable an increased current and brightness without increasing the current density or reducing the OLED lifetime. It is therefore also preferred that OLED pixels are large.
Inorganic light-emitting diode (iLED) displays are used in public spaces and are viewed at a wide range of distances by different viewers in the public space. Displays using micro-iLEDs (for example having an area less than 1 mm square, less than 100 microns square, or less than 50 microns square or having an area small enough that it is not visible to an unaided observer of the display at a designed viewing distance) are also described. For example, U.S. Pat. No. 8,722,458 entitled Optical Systems Fabricated by Printing-Based Assembly teaches, inter alia, transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate. Inorganic light-emitting diodes are commonly made in large quantities on semiconductor substrates. However, variability in materials and process result in iLEDs that have a corresponding variability in color, brightness, and efficiency.
Full-color displays have pixels that include multiple sub-pixel light emitters that each emit different colors of light. The different colors define the color gamut of the display and are intended to be viewed as a single emissive element having a color defined by the combination of colors emitted by the individual sub-pixel light emitters. The resolution of the display is specified by the number of pixels in the display per linear metric, for example pixels per inch. However, when viewed at ranges closer than the designed viewing distance for the display, for example in interior public spaces, the individual pixels can be distinguished and the individual color sub-pixel light emitters can be perceptible, increasing display pixelation and decreasing viewer satisfaction with the display. Furthermore, fixed pattern noise, such as the screen-door effect, can become visible.
There is a need, therefore, for inorganic light-emitting diode displays that provide improved color mixing and improved brightness and provide reduced pixelation, reduced fixed-pattern noise, and reduced variability in color, efficiency, and brightness.